After believing that his son is possessed, a former sinful man learns that the spirit is son’s mother whom the man had shot dead a few years ago and she wants revenge

This gives me a “What Lies Beneath” vibe - action via the (protag) possessed wife. Consider rearranging your logline to show from the possessed son's POV and keep the how and who is possessing him as part of your plot twists.

Keep going!

This logline doesn't have an action. Who is the protagonist? I'm assuming it's the sinful man but the logline reads as though the mother could be the protagonist. If it's the man that's the protagonist what does he do once his wife possesses his son?

Also I wouldn't use the word "believing" in the logline. I would just say something like "After a reformed sinner's son is possessed by the vengeful wife he murdered, he must find a way to exorcise her before the truth is revealed."